THE DISEASES OF PLANTS ppsx

THE DISEASES OF PLANTS THE CAUSE AND NATURE OF PLANT DISEASE Plants have diseases just as animals do; not the same diseases, to be sure, but just as serious for the plant.. This mold i

THE DISEASES OF PLANTS

THE CAUSE AND NATURE OF PLANT DISEASE

Plants have diseases just as animals do; not the same diseases, to be sure, but just

as serious for the plant Some of them are so dangerous that they kill the plant; others partly or wholly destroy its usefulness or its beauty Some diseases are found oftenest on very young plants, others prey on the middle-aged tree, while still others attack merely the fruit Whenever a farmer or fruit-grower has disease

on his plants, he is sure to lose much profit

You have all seen rotten fruit This is diseased fruit Fruit rot is a plant disease It costs farmers millions of dollars annually A fruit-grower recently lost sixty carloads of peaches in a single year through rot which could have been largely prevented if he had known how

Many of the yellowish or discolored spots on leaves are the result of disease, as is also the smut of wheat, corn, and oats, the blight of the pear, and the wilt of cotton Many of these diseases are contagious, or, as we often hear said of measles,

"catching." This is true, among others, of the apple and peach rots A healthy apple can catch this disease from a sick apple You often see evidence of this in the apple bin So, too, many of the diseases found in the field or garden are contagious.[Pg 123]

Sometimes when the skin of a rotten apple has been broken you will find in the broken place a blue mold It was this that caused the apple to decay This mold is a living plant; very small, certainly, but nevertheless a plant Let us learn a little about molds, in order that we may better understand our apple and potato rots, as well as other plant diseases

If you cut a lemon and let it stand for a day or two, there will probably appear a blue mold like that you have seen on the surface of canned fruit Bread also sometimes has this blue mold; at other times bread has a black mold, and yet again

a pink or a yellow mold

These and all other molds are tiny living plants Instead of seeds they produce many very small bodies that serve the purpose of seeds and reproduce the mold

These are called spores Fig 112 shows how they are borne on the parent plant

It is also of great importance to decide whether by keeping the spores away we may prevent mold Possibly this experiment will help us Moisten a piece of bread, then dip a match or a pin into the blue mold on a lemon, and draw the match across the moist bread You will thus plant the spores in a row, though they are so small that perhaps you may not see any of them Place the bread in a damp place[Pg 124] for a few days and watch it Does the mold grow where you planted it? Does it grow elsewhere? This experiment should prove to you that molds are living things and can be planted If you find spots elsewhere, you must bear in mind that these spores are very small and light and that some of them were probably blown about when you made your sowing When you touch the moldy portion of a dry lemon, you see a cloud of dust rise This dust is made of millions of spores

If you plant many other kinds of mold you will find that the molds come true to the kind that is planted; that like produces like even among molds

You can prove, also, that the mold is caused only by other mold To do this, put some wet bread in a wide-mouthed bottle and plug the mouth of the bottle with cotton Kill all the spores that may be in this bottle by steaming it an hour in a cooking-steamer This bread will not mold until you allow live mold from the outside to enter If, however, at any time[Pg 125] you open the bottle and allow spores to enter, or if you plant spores therein, and if there be moisture enough, mold will immediately set in

SHOWING HOW SPORES ARE BORNE

The little plants which make up these molds are called fungi Some fungi, such as

the toadstools, puffballs, and devil's snuff-box, are quite large; others, namely the molds, are very small; and others are even smaller than the molds Fungi never have the green color of ordinary plants, always reproduce by spores, and feed on living matter or matter that was once alive Puffballs, for example, are found on rotting wood or dead twigs or roots Some fungi grow on living plants,[Pg 126] and these produce plant disease by taking their nourishment from the plant on which

they grow; the latter plant is called the host

The same blue mold that grows on bread often attacks apples that have been slightly bruised; it cannot pierce healthy apple skin You can plant the mold in the bruised apple just as you did on bread and watch its rapid spread through the apple You learn from this the need of preventing bruised or decayed apples from coming

in contact with healthy fruit

THE SPORES ARE BORNE ON STALKS

Just as the fungus studied above lives in the apple or bread, so other varieties live

on leaves, bark, etc Fig 113 represents the surface of a mildewed rose leaf greatly magnified This mildew is a fungus You can see its creeping stems, its upright stalk, and numerous spores ready to fall off and spread the disease with the first breath of wind You must remember that this figure is greatly magnified, and that the whole portion shown in the figure is only about one tenth of an inch across Fig 114 shows the general appearance of a twig affected by this disease

Mildew on the rose or on any other plant may be killed by spraying the leaves with

a solution of liver of sulphur; to make this solution, use one ounce of the liver of sulphur to two gallons of water.[Pg 127]

The fungus that causes the pear-leaf spots has its spores in little pits (Fig 115) The spores of some fungi also grow on stalks, as shown in Fig 116 This figure represents an enlarged view of the pear scab, which causes so much destruction

You see, then, that fungi are living plants that grow at the expense of other plants and cause disease Now if you can cover the leaf with a poison that will kill the spore when it comes, you can prevent the disease One such poison is the Bordeaux

(bôr-dō') mixture, which has proved of great value to farmers

Since the fungus in most cases lives within the leaves, the poison on the outside does no good after the fungus is established The treatment can be used only

to preventattack, not to cure, except in the case of a few mildews that live on the

outside of the leaf, as does the rose mildew

EXERCISE

Why do things mold more readily in damp places? Do you now understand why fruit is heated before it is canned? Try to grow several kinds of mold Do you know any fungi which may be eaten?

Transfer disease from a rotten apple to a healthy one and note the rapidity of decay How many really healthy leaves can you find on a strawberry plant? Do you find any spots with reddish borders and white centers? Do you know that this is a serious disease of the strawberry? What damage does fruit mold do to peaches, plums, or strawberries?

Write to your experiment station for bulletins on plant diseases and methods for making and using spraying mixtures

YEAST AND BACTERIA

Can you imagine a plant so small that it would take one hundred plants lying side

by side to equal the thickness of a sheet of writing-paper? There are plants that are

so small Moreover, these same plants are of the utmost importance to man Some

of them do him great injury, while others aid him very much.[Pg 128]

You will see their importance when you are told that certain of them in their habits

of life cause great change in the substances in which they live For example, when living in a sugary substance they change the sugar into a gas and an alcohol Do you remember the bright bubbles of gas you have seen rising in sweet cider or in wine as it soured? These bubbles are caused by one of these small plants—the yeast plant As the yeast plant grows in the sweet fruit juice, alcohol is made and a gas is given off at the same time, and this gas makes the bubbles

Later, other kinds of plants equally small will grow and change the alcohol into an acid which you will recognize as vinegar by its sour taste and peculiar odor Thus vinegar is made by the action of two different kinds of little living plants in the cider That these are living beings you can prove by heating the cider and keeping

it tightly sealed so that nothing can enter it You will find that because the living germs have been killed by the heat, the cider will not ferment or sour as it did before The germs could of course be killed by poisons, but then the cider would be unfit for use It is this same little yeast plant that causes bread to rise

When you see any decaying matter you may know that in it minute plants much like the yeast plant are at work Since decay is due to them, we take advantage of the fact that they cannot grow in strong brine or smoke; and we prepare meat for keeping by salting it or by smoking it or by both of these methods.[Pg 129]

You see that some of the yeast plants andbacteria, as many of these forms are

called, are very friendly to us, while others do us great harm

Some bacteria grow within the bodies of men and other animals or in plants When they do so they may produce disease Typhoid fever, diphtheria, consumption, and

many other serious diseases are caused by bacteria Fig 118, e, shows the

bacterium that causes typhoid fever In the picture, of course, it is very greatly

magnified In reality these bacteria are so small that about twenty-five thousand of them side by side would extend only one inch These small beings produce their great effects by very rapid multiplication and by giving off powerful poisons

Bacteria are so small that they are readily borne on the dust particles of the air and are often taken into the body through the breath and also through water or milk You can therefore see how careful you should be to prevent germs from getting into the air or into water or milk when there is disease about your home You

should heed carefully all instructions of your physician on this point, so that you

PREVENTION OF PLANT DISEASE

In the last two sections you have learned something of the nature of those fungi and bacteria that cause disease in animals and plants Now let us see how we can use this knowledge to lessen the diseases of our crops Farmers lose through plant diseases much that could be saved by proper precaution.[Pg 130]

First, you must remember that every diseased fruit, twig, or leaf bears millions of spores These must be destroyed by burning They must not be allowed to lie about and spread the disease in the spring See that decayed fruit in the bin or on the trees

is destroyed in the same manner Never throw decayed fruit into the garden or orchard, as it may cause disease the following year

Second, you can often kill spores on seeds before they are planted and thus prevent the development of the fungus

Third, often the foliage of the plant can be sprayed with a poison that will prevent the germination of the spores

Fourth, some varieties of plants resist disease much more stoutly than others We may often select the resistant form to great advantage (see Fig 119)

Fifth, after big limbs are pruned off, decay often sets in at the wound This decay may be prevented by coating the cut surface with paint, tar, or some other substance that will not allow spores to enter the wound or to germinate there

Sixth, it frequently happens that the spore or fungus remains in the soil This is true

in the cotton wilt, and the remedy is so to rotate crops that the diseased land is not used again for this crop until the spores or fungi have died

SOME SPECIAL PLANT DISEASES

Fire-Blight of the Pear and Apple You have perhaps heard your father speak of

the "fire-blight" of pear and apple trees This is one of the most injurious and most widely known of fruit diseases Do you want to know the cause of this disease and how to prevent it?

First, how will you recognize this disease? If the diseased bough at which you are looking has true fire-blight, you will see a blackened twig with withered, blackened leaves During winter the leaves do not fall from blighted twigs as they

do from healthy ones The leaves wither because of the diseased twig, not because they are themselves diseased Only rarely does the blight really enter the leaf Sometimes a sharp line separates the blighted from the healthy part of the twig

This disease is caused by bacteria, of which you have read in another section The fire-blight bacteria grow in the juicy part of the stem, between the wood and the

bark This tender, fresh layer (as explained on page 79) is called the cambium, and

is the part that breaks away and allows you to slip the bark off when you make your bark whistle in the spring The growth of new wood takes place in the cambium, and this part of the twig is therefore full of nourishment If this nourishment is stolen the plant of course soon suffers

The bacteria causing fire-blight are readily carried from flower to flower and from twig to twig by insects; therefore to keep these and other bacteria away from your

trees you must see to it that all the trees in the neighborhood of your orchard are kept free from mischievous enemies If harmful bacteria exist in near-by trees, insects will carry them to your orchard You must therefore watch all the relatives

of the pear; namely, the apple, hawthorn, crab, quince, and mountain ash, for any

of these trees may harbor the germs

BACTERIAMagnified

When any tree shows blight, every diseased twig on it must be cut off and burned

in order to kill the germs, and you must cut low enough on the twig to get all the bacteria It is best to cut a foot below the blackened portion If by chance your knife should cut into wood containing the living[Pg 133][Pg 132]germs, and then you should cut into healthy wood with the same knife, you yourself would spread the disease It is therefore best after each cutting to dip your knife into a solution of carbolic acid This will kill all bacteria clinging to the knife-blade The surest time

to do complete trimming is after the leaves fall in the autumn, as diseased twigs are most easily recognized at that time, but the orchard should be carefully watched in the spring also If a large limb shows the blight, it is perhaps best to cut the tree entirely down There is little hope for such a tree

A large pear-grower once said that no man with a sharp knife need fear the fire-blight Yet our country loses greatly by this disease each year

It may be added that winter pruning tends to make the tree form much new wood and thus favors the disease Rich soil and fertilizers make it much easier in a similar way for the tree to become a prey to blight

EXERCISE

Ask your teacher to show you a case of fire-blight on a pear or apple tree Can you distinguish between healthy and diseased wood? Cut the twig open lengthwise and see how deep into the wood and how far down the stem the disease extends Can you tell surely from the outside how far the twig is diseased? Can you find any twig that does not show a distinct line of separation between diseased and healthy wood? If so, the bacteria are still living in the cambium Cut out a small bit of the diseased portion and insert it under the bark of a healthy, juicy twig within a few inches of its tip and watch it from day to day Does the[Pg 134] tree catch the disease? This experiment may prove to you how easily the disease spreads If you should see any drops like dew hanging from diseased twigs, touch a little of this moisture to a healthy flower and watch for results

Cut and burn all diseased twigs that you can find Estimate the damage done by fire-blight

Farmers' bulletins on orchard enemies are published by the Department of Agriculture, Washington, D.C., and can be had by writing for them They will help your father much in treating fire-blight

Oat Smuts Let us go out into a near-by oat field and look for all the blackened

heads of grain that we can find How many are there? To count accurately let us select an area one foot square We must look carefully, for many of these blackened heads are so low that we shall not see them at the first glance You will

be surprised to find as many as thirty or forty heads in every hundred so blackened

These blackened heads are due to a plant disease called smut

When threshing-time comes you will notice a great quantity of black dust coming from the grain as it passes through the machine The air is full of it This black dust consists of the spores of a tiny fungous plant The fungous smut plant grows upon the oat plant, ripens its spores in the head, and is ready to be thoroughly scattered among the grains of the oats as they come from the threshing-machine

These spores cling to the grain and at the next planting are ready to attack the sprouting plantlet A curious thing about the smut is that it can gain a foothold only

on very[Pg 135] young oat plants; that is, on plants about an inch long or of the age shown in Fig 121

When grain covered with smut spores is planted, the spores develop with the sprouting seeds and are ready to attack the young plant as it breaks through the seed-coat You see, then, how important it is to have seed grain free from smut A substance has been found that will, without injuring the seeds, kill all the smut

spores clinging to the grain This substance is called formalin Enough seed to

plant a whole acre can be treated with formalin at a cost of only a few cents Such treatment insures a full crop and clean seed for future planting Try it if you have any smut

Fig 122 illustrates what may be gained by using seeds treated to prevent smut The annual loss to the farmers of the United States from smut on oats amounts to several millions of dollars All that is needed to prevent this loss is a little care in the treatment of seed and a proper rotation of crops

EXERCISE

Count the smutted heads on a patch three feet square and estimate the percentage

of smut in all the wheat and oat fields near your home On which is it most abundant? Do you know of any fields that have[Pg 136] been treated for smut? If

so, look for smut in these fields Ask how they were treated Do you know of any one who uses bluestone for wheat smut? Can oats be treated with bluestone?

At planting time get an ounce of formalin at your drug store or from the state experiment station Mix this with three gallons of water This amount will treat three bushels of seeds Spread the seeds thinly on the barn floor and sprinkle them with the mixture, being careful that all the seeds are thoroughly moistened Cover closely with blankets for a few hours and plant very soon after treatment Try this